Piezoelectric transducers capable of generating pulses of ultrasonic energy, e.g., sound waves, in response to an electrical excitation are formed of one or more bodies of piezoelectric material, usually a ceramic such as Lead Zirconate Titanate (PZT), with electrodes on opposite sides for coupling to an electric power supply. Such transducers have many applications, including in the medical field. Among the medical applications are ultrasound imaging, lithotripsy, i.e., using ultrasound pulses to break up kidney stones or the like, and lysing or destroying the cell walls of adipose/cellulite tissue for cosmetic procedures, generally by causing cavitation in the tissue.
For such medical and cosmetic purposes, it is often desirable to be able to focus the ultrasonic output of the transducer. To achieve this, the transducers are often comprised of a cup-shaped piezoelectric ceramic shell and conductive layers forming electrodes covering the convex outside and concave inside of the piezoelectric shell. Typically, the transducers are hemispherical, with the “open end”, i.e., the equatorial plane positioned toward the subject being treated.
The transducer is excited to vibrate and generate ultrasound by pulsing it using an AC power supply generally operating at a resonant frequency of vibration of the piezoelectric material.
A hemispherical transducer in which the conductive surfaces define a single electrode pair exhibits an “axial focal pattern”. This is an ellipsoidal pattern having a relatively small cross section and relatively long axis coincident with a “longitudinal” axis of the transducer, i.e., a line through the center of rotation of the transducer perpendicular to the equatorial plane.